Analytical Industries GPR-35 Oxygen Analyzer - Manual (Page 20)

Advanced Instruments Inc.
20
Span Calibration
Span Calibration involves adjusting the transmitter electronics to the sensor’s signal output at a given oxygen standard.
Maximum drift from calibration temperature is approximately 0.11% of reading per °C. The frequency of calibration varies with
the application conditions, the degree of accuracy required by the application and the quality requirements of the user.
However, the interval between span calibrations should not exceed three (3) months.
Note: Regardless of the oxygen concentration of the standard used, the span calibration process takes approximately 10
minutes.
Recommendations General:
¾ The interval between span calibrations should not exceed three (3) months.
¾ Caution: Prematurely initiating the SPAN CALIBRATION function before the analyzer reading has stabilized can result in
erroneous readings. This is especially true when installing a new sensor that must adjust to the difference in oxygen
concentrations. It should take about 2 minutes for the sensor to equilibrate in ambient air from storage packaging.
¾ Always calibrate at the same temperature and pressure of the sample gas stream.
¾ For 'optimum calibration accuracy' calibrate with a span gas approximating 80% of the full scale range one or a higher
range than the full scale range of interest (normal use) to achieve the effect of “narrowing the error” by moving downscale
as illustrated by Graph A in the Accuracy & Calibration section.
¾ Calibrating with a span gas approximating 5-10% of the full scale range near the expected oxygen concentration of the
sample gas is acceptable but less accurate than ‘optimum calibration accuracy’ method recommended – the method usually
depends on the gas available.
¾ Calibrating at the same 5-10% of the full scale range for measurements at the higher end of the range (example:
calibrating an Oxygen Purity Analyzer in air at 20.9% oxygen with the intention of measuring oxygen levels of 50-100%)
results in the effect of “expanding the error” by moving upscale as illustrated by Graph A and Example 1 in the Accuracy &
Calibration section above and is not recommended. Of course the user can always elect at his discretion to accept an
accuracy error of +2-3% of full scale range if no other span gas is available.
¾ Calibrate ambient area monitors with a certified span gas or a clean source of instrument air.
Recommendations Air Calibration:
The inherent linearity of the galvanic fuel cell type oxygen sensor enables the user to calibrate any analyzer with ambient air
(20.9% oxygen) and operate the analyzer within the stated accuracy spec on the lowest most sensitive range available with the
analyzer – it is not necessary to recalibrate the analyzer with span gas containing a lower oxygen concentration.
¾ Avoid calibration of ambient area monitors with the surrounding atmosphere unless assured the oxygen content is 20.9%.
¾ When certified span gas is not available.
¾ When installing or replacing the oxygen sensor.
Required components: Refer to Installing Span Gas section above.
All electrochemical oxygen sensors respond to partial pressure changes in oxygen. The sensors
are equally capable of analyzing the oxygen content of a flowing sample gas stream or
monitoring the oxygen concentration in ambient air (such as a confined space such in a control
room or an open area such as a landfill or bio-pond).
The GPR-35 designed for in-situ ambient or area monitoring and has no sample system
because the sensor is intended to be exposed directly to the surrounding ambient atmosphere
which it is sampling and to operate at atmospheric pressure, however, slightly positive
pressure has minimal effect on accuracy.
A certified span gas is recommended for calibration purposes, part number A-2344 Flow Through Adapter is supplied with the
GPR-35 and provides: a hose connection for piping the calibration gas (see positive pressure see below) to the sensor, an o-ring
seal to isolate the sensor from the atmosphere being monitored and outlet to vent the calibration gas
Procedure Calibration with Span Gas or Instrument Air:
1. Use the following procedure regardless of the sensor housing included with the analyzer.
2. Refer to Span Gas Preparation above and regulate the pressure (30 psig) and flow rate (2 SCFH) as recommended.
3. Introduce a certified span gas of oxygen in nitrogen where the oxygen value is 20.9%.
4. Connect the 1/4'’ diameter plastic tubing by pushing it over the barbed fitting (P/N FITN-1029).